Ocular vascular and epithelial permeability play an important role in wound healing and other ocular homeostatic mechanisms. The presence of permeability barriers controls the movement of solutes between different ocular compartments (e.g., choroid and retina). A variety of electron-dense tracers are available, ranging in molecular weight from 1,900 to 500,000. After intravascular injection, the tracer is demonstrated by the appropriate chemical reaction which reveals its movement in the extracellular space. Microperoxidase and horseradish peroxidase have been used to demonstrate the normal pattern of tracer movement in the blood-aqueous, blood-retinal and chorioretinal barriers in primates. Experimental cryosurgery was used to alter barrier permeability and to determine the anatomic effects of such treatment. The principal activity proposed is the thorough ultrastructural study of the limbal capillary plexus and its permeability to ultrastructural tracers in rats, rabbits and primates. After the normal anatomy and tracer flow is studied, corneal vascularization will be induced by a variety of techniques. Permeability changes during the process of vascularization will be studied and factors relating to the mechanism of vasculogenesis will be sought.